Microorganisms such as bacteria are generally cultured on a culture medium (agar) contained in a Petri dish. Petri dishes are cylindrical dishes which are not very deep, consisting of a base and a lid, made of transparent material such as glass or a plastic (polystyrene). Microorganisms are generally detected by observation with the naked eye under uniform illumination; bacterial colonies generally appear in the form of more or less curved clusters of material, which can be seen through the walls or the bottom of the Petri dish. Observation through the lid is generally made difficult or impossible by the thin layer of condensation, made up of diffusing droplets having a diameter of approximately between 1 μm and 1 mm, which covers the internal surface of said lid. Thus, when the culture medium is scattering or absorbent (in the case of blood agars), it is necessary to open the Petri dish in order to observe the cultured microorganisms, but this leads to a risk of contamination.